According to JP-A-3-136219, a conventional ignition coil generates high voltage electric power, and the high voltage electric power is supplied to an ignition plug via a mechanical distributor and a high-tension cord. Presently, ignition coils are individually provided to cylinders of an internal combustion engine to directly supply high voltage power to ignition plugs. When the diameter of an ignition plug is reduced, the cross-sectional area of an engine water jacket arranged around an ignition plug can be increased, so that cooling efficiency of the engine can be enhanced. Therefore, the diameter of an ignition plug needs to be reduced in order to enhance fuel efficiency of a vehicle and to enhance engine power.
According to JP-A-3-136219, the conventional ignition coil has a structure, in which output electric power can be enhanced without jumboizing. That is, an ignition coil can be small sized applying the structure of the ignition coil, when output voltage is the same. The ignition coil includes a core, a permanent magnet, a primary bobbin, a secondary bobbin and a case. The core partially forms a closed magnetic passage, in which the permanent magnet is provided. A primary coil is wound on the primary bobbin. A secondary coil is wound on the secondary bobbin. The case receives the above components. The core is constructed of a first core and a second core that are made of silicon steel plates. The first core has a T-shaped cross-section, and the second core has an E-shaped cross-section in the radial direction. The permanent magnet is arranged between a radially central protrusion of the first core and a radially central protrusion of the second core to generate magnetic flux in an opposite direction as magnetic flux generated by the first coil. That is, magnetic flux generated by the first coil is reverse-biased by the magnetic flux generated by the permanent magnet. Therefore, magnetic flux passing through the closed magnetic passage is reduced by magnetic flux generated by the permanent magnet. However in this structure, magnetic flux generated by the primary coil does not change, and voltage induced in the secondary coil, i.e., output voltage of the secondary coil does not change. Accordingly, magnetic saturation can be avoided even the cross-sectional area of the closed magnetic passage is reduced. As a result, the diameter of the closed magnetic passage (magnetic circuit) can be reduced, while maintaining output voltage.
However, magnetic flux generated by the primary coil is substantially large in the ignition coil. By contrast, magnetic flux generated by the permanent magnet for reverse biasing in the magnetic passage is limited. Magnetic flux generated by the permanent magnet cannot be easily increased, because magnetic property of the permanent magnet cannot be easily enhanced and the size of the permanent magnet is limited. Accordingly, magnetic flux passing through the closed magnetic passage cannot be sufficiently reverse-biased for reducing the magnetic flux, and the ignition coil is difficult to be small sized.